The invention relates to radar area intrusion detection systems, and more particularly to radar intrusion detection systems which are capable of avoiding false alarms due to close-in targets, such as rain on the face of the radome, microphonic vibration of the radome, or small windblown articles.
Area intrusion detection devices are commonly used for protection of fenced outdoor regions and the like where equipment is protected or stored. Various microwave area intrusion protection devices are known. For example, see U.S. Pat. No. 4,328,487, issued May 4, 1982 and assigned to the present assignee.
Various modulation schemes have been used in low cost microwave short range surveillance systems. The state of the art is reviewed in "Modulation Schemes in Low-Cost Microwave Field Sensors" by Peter A. Jefford and Michael J. Howes, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-31, No. 8, August, 1983. Also see the paper "Microprocessor Target Assessment for Microwave Intruder Alarms" by Fabian Monds, Caesar Stewart, and Robert Kane, presented at the University of Kentucky, Lexington, Ky., May 12-14, 1982.
Present area instrusion detection microwave systems all have the shortcoming that they are unable to distinguish between small, very "close in" targets, such as raindrops on the face of a radome in front of the transmitting antenna, "microphonic" vibration on the surface of the radome in front of the antenna, and small windblown articles that pass immediately in front of the radome from larger, more distant targets, such as a burglar moving through the protected area at a distance of several hundred feet. Present area intrusion detection microwave systems also are incapable of distinguishing the burglar from large, distant moving targets, such as a large truck a thousand or more feet distant. As a result, the present area intrusion protection systems generate a sometimes unacceptably large number of false alarms due to the reflections or signals that are generated by the above-mentioned unintended targets. One of the main radar systems available is the continuous wave type, in which a carrier is continuously transmitted by the antenna to a target and the reflected return signal is mixed with a portion of the transmitted signals to produce a "doppler signal" which has a DC level produced in response to return signals from stationery targets and a time-varying level produced in response to return signals from moving targets. This technique has the difficulty of being sensitive both to (1) small close-in targets, such as raindrops on the surface of the radome, or vibration of the radome surface, and (2) very large, distant targets. Even though "umbrellas" or shields have been devised to prevent raindrops from accumulating on and running down the face of the radome in front of the antenna and producing reflections that trigger false alarms, these rain shields do not solve the problem in the presence of high winds which blow rain underneath the shiels onto the face of the radome.
Pulsed radar systems have been used for many years. Pulsed radar systems having "range gates" which "cut off" targets beyond a certain range have been utilized. However, all known pulsed radar systems have utilized very narrow transmitted "pulses" or bursts of the microwave carrier signal. The use of such narrow pulses or bursts provides a straight forward method of range discrimination, but has the shortcoming that the narrow pulses or bursts generate many side band frequencies in prohibited parts of the electromagnetic spectrum. Federal Communications (FCC) regulations permit such side band emissions in certain military and civilian applications, but Part 15, Subpart F of the FCC reguations restricts the bandwidth for area (or volumetric) intrusion detection radar systems. Therefore, use narrow pulsed radar systems has not heretofore been used in intrusion protection systems to which the present invention is directed.
Presently, there is no proposed technique for avoiding both the above-mentioned reflections from either very near, small, close in targets such as raindrops and very large, distant targets in the field of intrusion radar protection systems. Consequently, and where possible, users have simply "lived with" such reflections and the occasional false alarms caused thereby. The unavailability of a solution to this problem has limited the market for this type of radar intrusion detection system, especially in areas where false alarms are highly intolerable.
Therefore, it is clear that there is an unmet need for a low cost radar area or volumetric intrusion detection system which avoids false alarms set off by reflections produced by very large, distant moving targets or by very small, close targets, such as small wind-blown articles, raindrops on the face of the radome, and/or vibration of the surface of the radome.
Therefore, it is an object of the invention to provide a microwave intrusion detection system which avoids reflections or signals produced by small close-in targets.
It is another object of the invention to provide a low cost microwave intrusion detection system which avoids reflections or signals resulting from very small, close-in targets, including rain on the face of the radome and/or vibration of the radome without producing unacceptable spurrious radiation outside of the portion of the spectrum allocated by FCC regulations.
It is another object of the invention to provide a microwave intrusion detection system which avoids reflections or false alarm signals caused by both very small, close-in targets and/or very large, distant targets.